1. Field of the Invention
This invention relates to optical bistable devices and, in particular, to fast and low power optical bistable devices using bound excitons. Accordingly, it is a general object of this invention to provide new and improved devices of such character.
2. General Background
Optical bistability in a device comprising a thin cadmium sulfide platelet has been set forth in co-pending U.S Pat. application Ser. No. 6-713,510, entitled "Optical Bistable Devices", filed Mar. 19, 1985, by the applicants of this specification. That application, however, does not relate to optical cavities or to the intrinsic properties of bound excitons.
Also of interest are the following articles:
"Low power transverse optical bistability near bound excitons in cadmium sulfide", M. Dagenais and H. G. Winful, Appl. Phys. Lett. 44(6), 15 Mar. 1984, pp. 574-576.
"Cavityless optical bistability due to light-induced absorption in cadmium sulfide", M. Dagenais and W.F. Sharfin, Appl. Phys. Lett. 45(3), 1 Aug. 1984, pp. 210-212.
"Giant nonlinearities and low power optical bistability in cadmium sulfide platelets", M. Dagenais, Phil. Trans. R. Soc. Lond. A., 28 June 1984, 4 pages.
The subject matter of the foregoing three publications, although of interest, do not relate to devices in a cavity.
Appearing in the XIII International Quantum Electronics Conference Technical Digest, June 18-21, 1984, Anaheim, Ca., pp. 62-63, is a summary entitled "TuEE4 Low-Power Optical Bistability in Cadmium Sulfide Platelets", M. Dagenais, W. F. Sharfin and H.G. Winful, which reports the observation of transverse and whole beam optical bistability in thin uncoated cadmium sulfide platelets. However, there, the observed whole-beam bistability is thermally induced and it is cavityless.
It has been noted that some of the largest nonlinearities in nature have been observed near the band gap of semiconductors. In a semiconductor, the processes that lead to a nonlinear optical response include the creation of free carriers (free electrons and holes), free excitons (moving Coulomb correlated electron-hole pairs), and biexcitons (free moving excitonic molecules). Many of these processes have already been used to demonstrate optical bistability. In past approaches, free excitons, that is, those that are not bound, have been used to demonstrate optical bistability. Mobile species were created that implied that the switching energy of a bistable device does not necessarily decrease by focusing the incident light more sharply. In the case of the bound excitons, concerning localized nonlinearity, switching energy can be reduced by sharply focusing the light beam, thus providing the possibility of dealing with large two-dimensional arrays (possibly 10.sup.4 .times. .sup.4 in a 1 cm.sup.2 area) that can be addressed at relatively low power. The foregoing is important for applications in two-dimensional signal processing. In addition to having a localized nonlinearity that decays mostly radiatively and very rapidly, the radiative lifetime of the I.sub.2 bound excitons have been measured to be 500 ps. Thus, very fast switching time, in the order of less than 1 ns, is expected in such a system. As the decay is mostly radiative, only a small amount of energy is dissipated nonradiatively as heat, in contrast to the nonlinearities due to mobile species in semiconductors. For these nonlinearities, a large fraction of the absorbed energy is dissipated nonradiatively.